This invention relates to an amplifier and, in particular, to an amplifier that drives a device by creating a high voltage waveform similar to a low voltage control signal and applying the waveform directly to the device.
A piezoelectric actuator requires high voltage, greater than typical battery voltages of 1.5 to 12.6 volts. A “high” voltage is 20-200 volts, with 100-120 volts currently being a typical drive voltage. Some line driven power supplies for actuators provide as much as 1000 volts. Producing high voltage from a battery is more difficult than producing high voltage from a power line.
A voltage boost converter can be used to convert the low voltage from a battery to a higher voltage for the driver. In a boost converter, the energy stored in an inductor is supplied to a capacitor as pulses of current at high voltage.
FIG. 1 is a schematic of a circuit including a known boost converter; e.g. see U.S. Pat. No. 3,913,000 (Cardwell, Jr.) or U.S. Pat. No. 4,527,096 (Kindlmann). Inductor 11 and transistor 12 are connected in series between supply 13 and ground or common. When transistor 12 turns on (conducts), current flows through inductor 11, storing energy in the magnetic field generated by the inductor. Current through inductor 11 increases quickly, depending upon battery voltage, inductance, internal resistances, and the on-resistance of transistor 12. When transistor 12 shuts off, the magnetic field collapses at a rate determined by the turn-off characteristic of transistor 12. The rate of collapse is quite rapid, much more rapid than the rate at which the field increases. The voltage across inductor 11 is proportional to the rate at which the field collapses. Voltages of one hundred volts or more are possible. Thus, a low voltage is converted into a high voltage by the boost converter.
When transistor 12 shuts off, the voltage at junction 15 is substantially higher than the voltage on capacitor 14 and current flows through diode 16, which is forward biased. Each pulse of current charges capacitor 14 a little and the charge on the capacitor increases incrementally. At some point, the voltage on capacitor 14 will be greater than the supply voltage. Diode 16 prevents current from flowing to supply 13 from capacitor 14. The voltage on capacitor 14 is the supply voltage for other components, such as amplifier 21.
The output of amplifier 21 is coupled to piezoelectric actuator 22. The input to amplifier 21 can receive an alternating current signal, for bi-directional movement, or a direct current signal, for unidirectional movement or as half of a complementary drive (two amplifiers, one for each polarity, coupled to opposite terminals of piezoelectric actuator 22). In a complementary drive, the absolute magnitudes of the boosted voltages are greater than the absolute magnitude of the battery voltage. A complementary drive can use half the high voltage (or be provided with twice the high voltage) of a single drive but may require two boost converters.
It is known in the art to generate low voltage waveforms from pulse width modulated (PWM) signals; e.g. see U.S. Pat. No. 4,914,396 (Berthiaume), U.S. Pat. No. 5,703,473 (Phillips et al.), and U.S. Pat. No. 5,994,973 (Toki). Dealing with high voltages makes difficult, and more expensive, manufacturing devices that must isolate and control such voltages. A high voltage amplifier introduces losses that further reduce efficiency. The storage capacitor takes up valuable board space and the design of the driver illustrated in FIG. 1 is not readily adapted to different applications.
As used herein, “similar” in waveform does not mean an exact replica but a close approximation.
In view of the foregoing, one embodiment of the present invention eliminates the storage capacitor in a haptic driver.